The German U-boat command had an unprecedented concentration of U-boats in the North Atlantic at that time, and on 14 March, it set about forming three large packs from boats that had been operating against the convoys SC.121 and HX.228. Groups Raubgraf (8 boats) and Stürmer (18 boats) were to operate against SC.122, and group Dr?nger (11 boats) was deployed against HX.229.
On 16 March, the first U-boat made contact with HX.229. Both convoys were sailing close to each other, and HX.229 was closing on the slower SC.122, which had already passed the Raubgraf patrol line undetected. The Raubgraf boats, as well as 11 boats of the Stürmer group, were thus deployed against HX.229 in the mistaken belief that it was SC.122. Inadequately defended by only 2 destroyers and 2 corvettes, HX.229 suffered a heavy mauling by the packs during the night of 16–17 March. The same night, U-boats of the Stürmer group made contact with SC.122. Realizing that the two convoys were about to merge, the German U-boat command committed the remainder of its 40 available U-boats within range to the battle. Throughout 17 March, long-range B-24 Liberator bombers from Iceland and SC.122 escorts with high-frequency direction-finding (HF/DF) equipment succeeded in fending off the contact-keeping boats. Only 1 U-boat managed to close for an attack, sinking 2 ships out of SC.122 on that day.
On 18 March, air cover provided by the Liberators of the Number 120 Squadron again prevented 21 of the 30 U-boats deployed against HX.229 from reaching the scene, and again, only 1 U-boat succeeded in closing for a daylight attack. A reinforcement of the surface escort group prevented serious losses during the night of 18–19 March, in which the U-boats claimed only 2 further ships before intensified air cover, now flying out of the British Isles, forced them to desist. Two U-boats were damaged and 1 was sunk before Grossadmiral (grand admiral) Karl Dönitz called off the operation on 20 March.
The tally of 21 Allied ships sunk, totaling 141,000 tons of shipping, as well as 1 destroyer lost during this largest convoy battle of the war was impressive, yet it was also deceptive. It had been achieved primarily because the sheer numbers of U-boats had saturated the convoy defenses. Nevertheless, only 16 of the 40 U-boats deployed against both convoys had actually been able to make contact, and owing to the diligence of the convoys' hard-pressed air and sea escorts, only 9 succeeded in torpedoing ships. Of the 39 U-boats that survived the battle, 16 subsequently required more than 40 days of maintenance due to damage sustained during the battle. Committing virtually all available North Atlantic boats to four convoys—SC.121, SC.122, HX.228, and HX.229—also meant that the other four eastbound North Atlantic convoys in March 1943 made their passage entirely unmolested.
Blair, Clay. Hitler's U-Boat War: The Hunted, 1942–1945. New York. Random House, 1998.; Boog, Horst, Werner Rahn, and Reinhard Stumpf. Der Globale Krieg: Die Ausweitung zum Weltkrieg und der Wechsel der Initiative. Vol. 6 in the series Das Deutsche Reich und der Zweite Weltkrieg, ed. Militärgeschichtliches Forschungsamt. Stuttgart, Germany: Deutsche Verlags-Anstalt, 1990.; Roskill, S. W. White Ensign: The British Navy at Way, 1939–1945. Annapolis, MD: Naval Institute Press, 1960.